Thursday, 16 September 2010

A subterranean way of life

Rake through a piece of woodland leaf litter with your hand and you may well come across a cobweb-like mat of fine white threads. These are fungal hyphae; they are building blocks of filamentous fungi, thin cylinders that contain cell nuclei, mitochondria and other life-defining organelles. The fungal fruiting bodies familiar to us as mushrooms and toadstools are formed from hyphae and great empires of these threads carpet the soil and shroud the rootlets of trees and shrubs.

Fungal hyphae show certain similarities to the roots of plants, not least in that they absorb nutrients from their surroundings, acquiring resources to support growth and, ultimately, the production of the fruiting bodies that will disperse their spores. The hyphae also produce digestive enzymes, which digest the material in which the hyphae are growing. Most typically these enzymes digest cellulose – think of all the fungi you have seen growing on dead wood, but others digest keratin (hair and fur) or chitin (the material that forms the exoskeletons of insects). There are even fungi (in the broadest sense) that produce enzymes which can digest kerosene!

Many of our familiar fungi are associated with trees; rather than occurring loosely in the soil, their hyphal networks accumulate around the rootlets of certain trees and larger shrubs. Some of the hyphae grow into the rootlet, forcing their way between the tree’s cells. This association is not as one-sided as you might imagine, since it is not simply a case of the fungus attacking and feeding on the tree’s tissue. While the fungus receives moisture and nutrients from the tree (the latter most likely as by-products) the tree receives access to the soil nutrients being harvested by the fungus. The structure of this close association, the rootlet and the hyphae, is known as a mycorrhiza or ‘fungus-root’.

Knowledge of fungus-tree associations can be particularly useful when seeking to find or identify certain fungi. There are, of course, a number of species that are generalist in their habits, and which are associated with a number of different tree species, but others are rather choosy. Fly Agaric, for instance, seems to favour mature birch trees (invariably those of 15 years age or older) or, to a lesser extent, pines. However, other features probably also have a roll to play, notably soil pH, soil condition and how wet the soil is. The sight of fruiting bodies at this time of the year reveals the presence of fungi that exist beyond our gaze for the rest of the year. The weather seems to have been favourable this year and there is a rich crop of fungi to be found across the woods, heaths and grasslands of Norfolk.

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